To date, research on the effects of urbanization, which include reduced biodiversity, has focused on changes at particular sites or along gradients of urbanization. Comparatively little work has investigated changes in biodiversity at any citywide—much less global—scale, and no attempt has been made to quantify such changes in human terms. We have developed a novel data set that reveals a systematic pattern of biodiversity: Within cities worldwide, most residents are concentrated in neighborhoods of impoverished biodiversity. This pattern exists despite substantial biodiversity present in cities overall, and becomes more severe when only native species are considered. As humanity becomes increasingly urban, these findings have a tragic and seldom-considered consequence: Billions of people may lose the opportunity to benefit from or develop an appreciation of nature. Because nearby surroundings shape people's baselines of ecological health, our findings suggest adverse consequences for conservation in general as well as for humans' quality of life if the problem remains uncorrected.